Energy storage technologies, particularly batteries, are key to providing independent electricity access where the grid is unavailable or unreliable, as well as enabling renewable energy on the electricity grid, and powering electric and hybrid vehicles. In grid and off-grid applications, such technologies are typically combined with solar photovoltaic (PV) systems. Currently, lead acid batteries are the most common technology for off-grid energy storage applications due to their low cost. However, lead acid batteries have low energy density (on the order of 40 Wh/kg), a short lifetime (100-800 cycles) and high environmental impact if hazardous lead is released as a consequence of inadequate handling or disposal. By comparison, for example, lithium ion (Li-ion) batteries have a high energy density (approx. 130 Wh/kg) and a long cycle life (>2000 cycles).
Rechargeable electrical energy storage devices such as batteries and supercapacitors come in a large variety of forms and types (e.g. chemistries). For example, amongst batteries, lithium ion (abbreviated Li-ion) technology is amongst the highest performing and is currently the most prevalent in commercial rechargeable batteries. However, even within this sub-category there are still multiple chemistries such as lithium cobalt oxide (LiCoO2), lithium iron phosphate (LFP), lithium manganese oxide (LMO), lithium nickel manganese cobalt oxide (NMC), lithium nickel cobalt aluminum oxide (NCA) and lithium titanate (LTO). Other popular rechargeable cell chemistries include lead acid, Nickel-Cadmium (Ni-Cad) and Nickel metal hydride (NiMH).
As technologies have developed (and continue to develop), the cell chemistries have changed and capacities and power capabilities have increased (and will continue to increase). Each cell chemistry and capacity has its own characteristic voltage vs. state of charge (SOC) curve that the cell exhibits between fully charged and fully discharged states.
Supercapacitors, also known as electric double-layer capacitors or ultracapacitors, also function as reversible electrical energy storage units or cells. Unlike rechargeable batteries, supercapacitors feature low energy densities but have high power capabilities and are therefore typically used to meet short-term high power demands. The combination or hybridization of rechargeable batteries and supercapacitors is desirable for applications that require both fast charging and discharging as well as high energy storage capabilities, with maximum battery lifetime. Such applications include, for example, electric and hybrid electric vehicles and stationary energy storage devices required to charge and discharge at high rates. Although desirable, the combination of rechargeable batteries and supercapacitors is difficult due to their different characteristic voltage ranges, power capabilities, and energy storage capacities.
Both supercapacitors and rechargeable batteries may be referred to as cells. In the discussion that follows, specific examples may be given that refer to batteries and/or supercapacitors, but the principles discussed may be applied to any electrical energy storage cell, including but not limited to batteries or supercapacitors.
Individual cells are frequently combined together to form packs, either to increase the voltage output of the pack by combining cells in series or to increase current by combining cells in parallel. Commercial battery or supercapacitor packs always combine cells of the same chemistry and nominal capacity so that each cell exhibits substantially the same voltage-SOC curve. Keeping the individual cells as close as possible in characteristics such as impedance and capacity improves the performance of the pack during charge and discharge, and particularly over many such charge/discharge cycles. Large differences in cell capacity are problematic for series connections, since the same current passes through all cells and the total battery capacity is limited by the cell with the lowest capacity. For parallel connections of cells, differences in cell voltages are problematic, since all cells are tied to the same voltage and the total pack voltage is constrained by the cell with the lowest voltage limit. Cell voltages can vary significantly for different cell chemistries and types.
Over the lifetime of a cell (over many charge and discharge cycles), the capacity of the cell is gradually reduced. This reduction in capacity happens at different rates for individual cells in a battery pack, and thus the individual cells in a battery pack become mismatched.
The performance of a battery pack as a whole is limited by the performance of its weakest cell. There are safety issues with over-charging and over-discharging any individual cell which means that once a single battery cell has been exhausted, the whole pack is deemed unusable, even though some cells may still have some useable capacity. Similarly, when a single cell has reached full charge, the supply of charge current must be stopped to the whole pack to avoid over-charging, even though some cells may not yet be fully charged. Similar concerns apply to supercapacitor packs. To ensure safety, management electronics must be provided to prevent over-charge and over-discharge conditions. Such battery management systems (or supercapacitor management systems, or more generally energy cell management systems, all of which will be covered here by the abbreviation BMS normally applied to Battery Management Systems) typically involve voltage and/or current monitoring and temperature sensors to detect overheating, over- or under-voltage and over-current, and to prevent charging in adverse conditions such as low temperatures (for example Li-ion cells may exhibit lithium plating if charged at low temperatures, which could result in internal short-circuiting).
In relation to rechargeable batteries, the above problem has been addressed by various cell balancing techniques that for example either detect when a cell has depleted and redirect charge from other cells that still have capacity to the depleted cell (balancing during discharge), or detect when a cell has reached full charge and then redirect charge from it either to an arbitrary waste load or in more complex systems to other cells that are not yet fully charged (balancing during charge). With these techniques, better use can be made of the capacities of all cells within a pack. However, these balancing techniques can be expensive, slow, and energy inefficient. Furthermore, they are designed to balance cells of similar type and capacity, and are thus unsuitable for cells of different types and capacities.
Battery packs for portable equipment such as laptop computers exist in large numbers and due to the short product lifetime the battery packs become redundant and are discarded regularly. Hundreds of tonnes of Li-ion batteries are disposed of every year. It has been found that the capacity of cells recovered from the same device may vary significantly. Many of the cells from such discarded battery packs still have a useful capacity even though the battery pack as a whole is at its end of life due to the performance of weaker cells. For example, one study found that around 50% of discarded cells had >70% of their nominal capacities. However trying to match these individual cells by chemistry and capacity to form new usable battery packs is not a cost effective process.
The cost of new battery packs makes them unsuitable for use in the developing world. The high cost of new battery packs is due to the cost of the cells as well as the cost of the charging/discharging electronics.